Metal oxide semiconductor (MOS) sensors are widely used in controlled combustion, toxic and inflammable gas leakage detection, and temperature measurements. The requirements for the on-site detection of hazardous gases call for integrated sensors with low energy consumption, fast response and rapid recovery. More importantly, the sensor should be able to recognize the type of gas that induces the response, and exhibit rapid sensing and subsequent recovery, thus behaving like a smart “electronic nose.” These considerations impose even more stringent requirements for sensor elements and their integration within the established microelectronics platforms. Such smart sensors are essential in numerous industrial, domestic and warfare environments including chemical industries, pollution monitoring, food quality control and chemical weapons detection.
The advent of nanostructured materials exhibiting enhanced and unusual physical and chemical properties, and the means to fabricate or pattern structures at the nanoscale, have paved the way for new and improved biological and chemical sensing and detection. As a result, the fabrication of miniaturized sensors using emerging nanomaterials has been an active topic in sensor research. One-dimensional nanostructured elements such as nanowires (e.g. carbon nanotube, silicon nanowire, semiconductor nanoribbon and mesowire) have been used to detect biological molecules and industrial gases. However, the necessity of separate steps in the synthesis and purification of nanowires and sensor fabrication requires additional manipulation to incorporate nanowires into electronic circuitry. Furthermore, it is very difficult to control the position and orientation of nanowires when using direct deposition techniques to pattern nanowire suspensions onto substrates.
External manipulations using atomic force microscope (AFM) and electrophoresis of nanowires inside a suspension have been used to increase the efficiency of nanowire bridging on electrode gaps. Another improved approach is the use of microfluidics to align a multitude of nanowires at the same time, followed by the deposition of electrodes across the desired nanowires. However, such “reverse” construction approaches require access to expensive and sophisticated facilities for nanowire observation and measurement of electrode deposition, and they are generally time-consuming with questionable batch-to-batch reproducibility. Furthermore, as far as integrated sensor construction is concerned, it remains a formidable challenge for parallel fabrication methods such as microfluidics to create sensor arrays with multiple detection capability using different sensor materials.
Thus, there exists a need for an effective and efficient approach for the nanopatterning of nanoporous sensor materials, and the subsequent fabrication of gas-sensing miniaturized nanodisk sensors.